So Im having troubles with my 06 h3. Truck started rough idling / loss of power and the check engine codes show misfire on cylinder 1 and low threshhold cat converter. I just replaced the cat with a universal magnaflow weld in. Dood did it half assed as I can see exhaust coming out around it and the welds dont go all the way around. So I know I have an exhaust leak around it but I havent had any problems and no check engine lights and the truck has run fine for several months. Should I start with the cat then replace plugs and clean MAF or should I start with the plugs? Or is there something else I should look at? I jave 190k on the odometer and the plugs just got changed at 170k

 

Fyi i get codes p0420 and p0301

 

I would think it is possible to make the connection of the PO420 to your leaking Cat. Is it leaking at both ends? Leaks can cause gas pulsations that could trigger a code if the anomaly is significant and long enough in duration. I would have that welded all way around, or sleeved at the joints if you can't get a weld on the top side. Once that's done clear the code and see what happens.

However, that may not be source because the Cat in the exhaust is not monitored downstream. The mini Cat in the Exhaust Manifold is monitored and that may be near cooked at 190K miles??? So if the Cat code persists after your leak is corrected, I would say it relates to the mini Cat.

For the misfire, usually the easy way to check that is to clear the code, swap the #1 coil pack for say #2 (or whichever you want) and see if the misfire follows the #1 coil pack to the new cylinder. So in my example, if you then get a PO302 after the swap, you know that the coil pack need replacement.

While you have the intake off to swap coil packs, might as well clean the TB (with TB cleaner) and the MAF (with MAF cleaner).

Good luck.

 

Thanks I pulled the airbox off to get to the plugs to find there is standing water in the 1 cylinder plug hole. Looks like there is a service bulletin saying this happens sometimes. My coil is all corroded from the water. Going to replace the coil and plug and see if that fixes it. Hopefully its that simple. The cat code hopefully will clear once it stops misfiring. Ill probably take it to a muffler shop to get it welded all the way around